This invention relates to a method and a device for tracking sweet spots of a sweet spot unit used in a transmissive electronic display for displaying information, said method and device imaging light after modulation with the information by the display in a directed manner on to observer eyes of at least one observer in sweet spots.
This invention can be applied in monoscopic and/or autostereoscopic displays for one or multiple observers. A display device according to this invention allows images to be displayed optionally in a two-dimensional, in a three-dimensional mode or in a mixed mode. In this document, the term “autostereoscopic display” denotes a display device using which at least one observer can view three-dimensional images without any additional aids from a large number of positions to be chosen freely by the observer.
Seen in the direction of light propagation, the sweet spot unit of a display comprises an illumination matrix with a multitude of illumination elements which emit or transmit light, and imaging means with imaging elements. The imaging means image the light of activated illumination elements of the illumination matrix on to the eyes of at least one observer in the form of one or multiple sweet spots in more or less ideal bundles of parallel rays. For this, a multitude of illumination elements are assigned to each imaging element of the imaging means. Sweet spots are regions in which information provided on the information panel can be viewed at high quality.
The homogeneity of the information displayed on the information panel must be ensured in the sweet spots at all times, and cross talk of information among the eyes of an observer must be prevented when viewing three-dimensional contents. These conditions must continue to be fulfilled if the observer changes their position in the space in front of the display device, so that the observer is continuously provided with high-quality monoscopic or stereoscopic image contents. For example, in a monoscopic display used in a vehicle the driver is shown a route map while a passenger can watch a movie. Both persons should be able to move in a certain range without losing their specific information.
Further, it has been shown that disturbances and imaging defects are likely to occur already if the number of light sources activated for a defined sweet spot is only slightly too small or too large. For example, observers may experience cross talk between individual sweet spots, and the image quality, for example homogeneity and contrast, may deteriorate. The human eye perceives such changes very easily.
Autostereoscopic displays are expected to present high-quality three-dimensional scenes, but also to exhibit properties which are irrespective of the number of observers, such as free and independent mobility of observers and optional access to multiple representations in a two-dimensional and/or three-dimensional mode.
In order to be able to fulfil all those requirements to an optimal degree, a suitable tracking system will be necessary which provides information for subsequent processing in tracking devices and for stereoscopic display of information. Such a tracking system must be capable of continuously detecting observer movements in front of the display device in a viewing space which is as large as possible, so that each observer is always provided with their specific image information irrespective of their actual position. This makes great demands on the accuracy of the position finders, on the quality of individual elements of the display and on the imaging quality of the display as a whole.
There are tracking systems which use mechanical, optical and other means, or combinations thereof, for tracking. However, these systems suffer from disadvantages which adversely affect their accuracy or suitability in real-time applications. Their design is often voluminous, and the viewing space in which the observers can be provided with information is very limited. Moreover, the required computing time increases considerably the more factors are to be considered in the process from position detection to information provision.
The document WO 03/053072 A1 discloses a multi-user display with a tracking system which collects three-dimensional position information and with sequential presentation of stereoscopic images. The display comprises, one after another, a backlight which can be addressed three-dimensionally, a large-area imaging lens for focusing light on the observer and a light modulator as an image matrix. The backlight is composed of a multitude of two-dimensional light source arrays which are disposed one behind another in a multitude of planes. Illumination elements in one of the light source arrays of the backlight are determined for activation according to the actual observer position. This method also allows light source tracking with respect to the distance between one or multiple observers and the display. Three-dimensional position information of all observer eyes are detected dynamically, assignable illumination elements of the backlight are opened, and bundles of rays are focused on the respective right/left observer eyes in synchronism with the modulated images.
The disadvantage of this display device is its low brightness, because only the light of one locally selectable point light source is available for illuminating the entire image per observer eye, and because the inactive light source arrays in the optical path absorb part of this light. In addition to its voluminous design, this three-dimensional backlight is difficult to manufacture.
The document U.S. Pat. No. 6,014,164 describes an autostereoscopic display for multiple observers with a light source tracking system. Light sources, which are arranged in pairs per observer, can be moved in x, y and z direction such to track the changing positions of the observers with the help of a control system. This method allows to continuously providing the observers sequentially with three-dimensional scenes on the information panel. The disadvantages described above also apply to this system because it also employs the method of light source tracking. In particular, expensive tracking means are required, because the observer-specific light source pairs must be tracked individually for each observer. These means do not allow a flat design of the display device to be achieved.
Prior art tracking methods are only able to deliver specific information to observers who are situated at various positions in a stereoscopic viewing space with considerable restrictions. The tracking range and brightness of the image matrix are limited. Moreover, the displays are voluminous and require expensive means, including computing means, to realise the tracking. The more observers are situated at various positions in front of the display, the greater becomes the volume of data to be calculated and, consequently, the more increases the delay between position detection and actual provision of sweet spots. It has thus become common practice not to calculate a certain part of the data in real time, but to store pre-calculated data in a look-up table, and to call up and to process such data as required. Another drawback is the fact that the storage capacity of the system will be exhausted quickly as the number of observers rises.